第二十一组(建议用时:45分钟)Ⅰ.阅读理解letely frustrated by the blocking and for 18—19 miles was just keeping away from people and being held up,” one participant grumbled after the 2017 London Marathon.“I had to overtake a lot of people and ended up with bruised(淤肿的) forearms from all the elbows Some researchers believe that we can find the answers through a more familiar system in which jams appear—road traffic flow.Martin Treiber, of the Technical University of Dresden in Germany, has previously developed models for traffic flow.One of the first attempts to model traffic flow was made in the 1950s by James Lighthill and his collaborator Gerard Whitham of Manchester University.They considered the traffic as a kind of liquid flowing down a pipe, and looked at how the flow changes as the fluid gets denser(浓稠).At first the flow rate increases as the density increases, there’s a risk of jams, and the flow rate drops sharply.
Treiber’s model of a marathon uses this same principle that the flow rate first increases and then decreases as the density of runners increases, thanks to an sudden switch from free to crowded flow. He assumes that there is a range of different preferred speeds for different runners, which each maintains throughout the race.With just these factors, Treiber can calculate the flow rate of runners, knowing the 揷arrying capacity?s(承载能力) at each point on the route.(which means those at the back have to wait until those in front have moved forward).Others assign runners to various groups according to ability, 6,000 participants.The traditional route had to be changed in 2017,because the police were no longer willing to close a road to ensure that runners could cross safely.It could pass either over a 60m wooden bridge or through a tunnel.Treiber used his model to predict the likely blocking caused in the various options.The model predicted that a mass start would risk an overload of runners if the bridge were to be used.Only by moving the starting point further back from the bridge could the danger be avoided—and even then, if some of the numbers assumed in the model were only slightly inaccurate, there was still a risk of jams at the bridge.On the other hand, no dangerous blocking seemed likely for the tunnel route.The run organizers consulted Treiber’s team, and eventually chose this option.
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